The present invention relates to a method and a device for measuring the pump operating parameters of a diaphragm delivery unit.
In many areas where diaphragm pumps are used, it is necessary to be able to measure the pump operating parameters using measuring techniques. If the delivery unit is used for toxic or flammable liquids, then strict safety requirements must be satisfied. The operativeness of these systems is usually monitored by a process control system. In the event of a malfunction, the system switches over to a standby delivery circuit. The goal is to be able to detect a failure of the pump as early as possible. The diaphragm of a diaphragm delivery unit is the component having the highest failure rate. It is subject to periodic cyclical stress over its entire service life.
For predicting a malfunction of a pump diaphragm, it is proposed in WO 95/06205 to use an electrically conductive fiber made of polytetrafluoroethylene. It is embedded in a diaphragm layer made of the same material and acts as a measuring transducer. The fiber is disposed on the diaphragm such that it essentially covers the entire shaped surface of the diaphragm in the form of a spiral or a double spiral. The ends of the fiber are led out to the edge and are connected to an electrical measuring device. The measuring device measures the ohmic resistance of the fiber. As soon as the diaphragm layer shows evidence of fatigue or cracks, this is communicated to the fiber. The result is a tearing at the fiber and a consequence of this is a change in the conductivity of the fiber, which is detected by the measuring device. In this way, using this measuring transducer embedded in a diaphragm layer, a crack or an incipient break in the diaphragm can be detected using measuring techniques. The disadvantage is that it is very expensive to manufacture the conductive plastic fiber, as well as to embed it in the diaphragm layer. Also disadvantageous is the fact that the electrical conductivity only changes slightly as a result of expansion. The practical evaluation of the measuring signal therefore is limited to confirming damage. Only the intactness of the diaphragm can be established.
The present invention is based on the objective of creating a method and a device which permit the pump operating parameters of a diaphragm delivery unit to be measured.
In the method according to the present invention, the conductivity of a resistance device mounted on a diaphragm is measured as a time-dependant quantity through one pumping period, and this quantity is used in the measurement. In a device for carrying out the method, the technical problem underlying the present invention is solved by mounting an elastomer conductive loop on a diaphragm of a diaphragm delivery unit and by connecting it in a bridge arm of a Wheatston bridge and/or in series to a shunt, a change in the operating state of the diaphragm bringing about a temporal change in the measuring voltage in the bridge arm or at the shunt.
In one embodiment, the invention includes a method for measuring the pump operating parameters of a diaphragm delivery unit wherein the conductivity of a resistance device mounted on a diaphragm is measured at least over one pumping period as a time-dependent quantity and this quantity is used in the measurement. In another embodiment, the resistance device is constituted by an elastomer conductive loop, wherein the elastomer conductive loop is connected in at least one bridge arm of a Wheatston bridge, and the time-dependent quantity is obtained from a measuring voltage in the diagonal arm of the Wheatston bridge. In another embodiment, the elastomer conductive loop is connected in series to a shunt, and the time dependent quantity is obtained from the measuring voltage at the shunt. In yet another embodiment, the time-dependent quantity is obtained by forming an average value and is used for monitoring the diaphragm for damage.
The present invention is based on the idea of arranging a resistive measuring transducer on a diaphragm such that the momentary operating state of the diaphragm can be measured by the temporal curve of the conductivity. The measuring signal obtained in this manner then reflects the momentary operating state actually predominating on the diaphragm. The conductivity of this resistance device characteristically changes over each pump cycle as a function of load and stroke. When the diaphragm expands, the resistance device expands. As a result of this expansion, the device experiences a change in its length along with a simultaneous reduction in its cross section. This geometric change in the resistance device is proportionate to the electrical resistance of the measuring transducer, i.e., in response to an expansion, the resistance increases. Using an evaluating electronics, it is possible from this time-dependent quantity to measure such pump operating parameters as pumping frequency, number of pumping cycles, dynamic pressure curve in the delivery chamber, as well as the position of the neutral phases of the pump diaphragm. Evaluating the measuring signal, in conjunction with a process control system, makes possible a reliable monitoring of the functioning of the diaphragm delivery unit and therefore of the parts of a process that are critical for safety. For example, if the diaphragm is operated mechanically by a drive flange, it is also possible to detect a malfunction in which the drive is active but the connection between the diaphragm and the drive flange has long since been severed. Using an appropriate device for processing the measuring data, it is also possible to record operating states that occurred in the past.
It is advantageous if the resistance device is formed by a surface conductive loop which is made of a conductive plastic. Materials of this type can be impressed onto the surface of a diaphragm layer in a simple manner using screen printing. In an electrically conductive elastomer, its conductivity can be predetermined by the quantity of electrically conductive fillers. Especially well-suited for a resistance device is a surface conductive loop made of an electrically conductive elastomer. The material of this elastomer conductive loop has a specific electrical resistance of less than 300 ohm/cm. The elastomer conductive loop can be manufactured in a simple manner by adding conductive soot or conductive graphite to the elastomer as a filler. In this manner, the sensor does not function so as to generate interference in response to a motion of the rubbery-elastic diaphragm plate. The measuring transducer can be designed so that its tensile strength lies between 8 and 25 N/mm2, its elongation at break lies between 50 to 400%, and the specific resistance in this context is less than 300 ohm/cm. Compared with other conductive plastics, such as are used for electrically conductive floors, the electrical resistance of the elastomer conductive loop is low. In this resistance range, a change in the conductivity is easy to measure.
It is advantageous to connect the surface conductive loop in at least one bridge arm of a Wheatston bridge and to obtain the time-dependent quantity in the diagonal arm of the bridge. In this manner, the pump operating parameters can be measured with great sensitivity.
A very simple measuring arrangement is obtained if the surface conductive loop is connected in series to a voltage source and to a shunt, and if the time-dependent quantity is obtained by measuring the voltage drop in the shunt.
During the operation of the pump, the time-dependent quantity is a pulsating quantity having a periodic curve. The measuring quantity is made up of a steady component, onto which an alternating component is superimposed. The pumping frequency can be measured in a simple manner by measuring the pumping frequency from the alternating component of the measuring signal, for example, using a bistable flip-flop.
From the alternating component of the time-dependent quantity, the delivery pressure can also be measured. The elastomer conductive loop is in contact with the diaphragm and is bonded to it. Both are rubbery-elastic and have similar mechanical properties. An expansion in the diaphragm brings about, as a consequence, an expansion in the elastomer conductive loop to the same degree. Thus the momentary value of expansion in the diaphragm corresponds to the momentarily predominating delivering pressure in the working chamber of the pump. From this signal, together with the pump frequency, a process control system can calculate the delivery performance, i.e., a quantitative stress placed on the diaphragm in the past. In this manner, it is possible to determine a favorable time for a diaphragm change. In addition, of course, operating parameters such as dynamic pressure curve, pumping frequency, number of cycles during one operating time of a long-duration recording are also available.
The monitoring of the diaphragm for damage can take place in a simple manner by generating an average value for the time-dependent quantity. Through comparison with a preestablished threshold value, a diaphragm break can be signaled.